Variable volume pipettes have been known in the prior art. For example, the pipette described in U.S. Pat. No. 3,640,434, issued on 2/8/72, uses a plurality of (three) differently-sized and integrally connected pistons, reciprocating in differently (but appropriately) sized piston chambers, to create one of three different aspirating and dispensing volumes. These volumes function using only one size dispensing tip removably mounted on a single mounting surface. The effective volume is determined by a complex valving member that selectively opens, or cuts off, one, two or three of the vent paths emanating from the piston chambers. The valving member is actuated by rotating a sleeve to open or cut off the vents. The problems with such a device include, (a) the valving member's complexity can lead to malfunctions, and (b) the user must determine which setting the sleeve is on, since this is not automatically determined by any step in preparing the pipette Inadvertence, negligence, or even distraction on the part of the user can lead to an inappropriate volume being used.
What has been desired, therefore, prior to this invention, is a variable volume pipette in which the selection of the pipette's volume is more positively, and yet more simply, controlled by the use of the pipette to avoid malfunctions and inappropriate use of the pipette.